I need set/get VAt number in shipping/billing address, and when the user checkout the order, but magento api don't have this value. how can i get it from php or Magento api?.
If I remember correctly the VAT number is on the customer record and not the address record. Because of this you have to get that customer record over the API, or, alternatively, you can move the TAX/VAT number to the shipping/billing address. There are various ways to achieve this, you could spend some time creating a clever module or you could just do something like re-use the FAX field, change a couple of translation strings and call the job done.